This invention relates generally to temporary and provisional dental crowns, and more particularly to flexible dimension crown shells and methods of making temporary and long-term provisional dental crowns using such shells.
Presently, there are three primary methods for fabricating temporary and provisional crowns. In a first technique, conventional prefabricated crown forms or shells, made of a metal such as aluminum or stainless steel, or of a polycarbonate such as the Ion crown forms sold by 3M Corporation, are trimmed and shaped to fit a prepared tooth. Examples of temporary crowns of this type are disclosed in U.S. Pat. No. 4,015,332 (Manne), U.S. Pat. No. 4,678,435 (Long), U.S. Pat. No. 4,778,386 (Spiry), and U.S. Pat. No. 5,458,489 (Tennyson).
A second technique calls for making an impression of the tooth before the tooth is prepared for a crown. After the impression is made, the tooth is prepared and the impression, filled with a bis-acryl material, is placed over the prepared tooth. After the bis-acryl material sets, it is removed from the dental impression, and then trimmed, polished, and seated in the mouth.
A third primary technique, which is used and recommended by Gordon Christenson, is also popular. According to the third technique, a putty-like ball of polymethyl-methacrylate is applied over a prepared tooth. The patient then bites down and the material begins to set. Before it completely sets, the putty-like material is removed from the tooth, trimmed and placed back on the tooth. Once the material sets, it is then trimmed again and the bite adjusted. Finally, the temporary crown is cemented to the tooth. In a variation of this technique, as disclosed in U.S. Pat. No. 5,385,469, a tubular dental form for forming a universal crown in situ is used.
Each of these techniques has various advantages and disadvantages. Using prefabricated forms, as in the first technique, for example, is fast and simple, but the fit of the conventional shell is not very good. The margins, in particular, do not fit well. Specifically, it is hard to get good proximal contact to adjacent teeth, and the contours and occlusion are not always good. Some manufacturers try to overcome these drawbacks by proliferating sizes and shapes of shells, with some selections providing as many as 80 different sizes and shapes of molars and bicuspids. Unfortunately, this attempted solution is expensive in terms of materials and also in terms of the time required for the dentist to pick the right shell.
Manne adds a degree of freedom to the first technique by providing an incisor shell that has slits in the mesio-distal sides to permit the shell to flex in the labio-lingual direction about a hinge axis at the occlusal surface. Long also provides a degree of freedom in this technique by having the mesio-distal sides of a temporary molar crown open to permit the acrylic resin filler material to protrude proximally to contact adjacent teeth. These shells, like others used in this technique, require trimming the free edges of their buccal and lingual sidewalls to ensure a good fit along the gingival margins, as well as a good occlusion. The shells in Long also appear to require substantial trimming of the filler material due to their open mesio-distal sides. Such trimming and fitting is time-consuming for the dentist and the patient.
The second technique gives good contours and bite accuracy, but making an impression is time-consuming. Furthermore, neither the strength nor the durability of temporary crowns produced by this technique are very good. Furthermore, the impression cannot be made if the patient's tooth is already broken when initially treated. The third technique, namely, free-forming a temporary crown of putty-like material, can be accurate and fairly fast compared to the other techniques, but only if performed by a skilled dentist or technician. It, too, however, is more time-consuming than desirable. Another main problem with this approach is that it is very technique-sensitive. A dental technician must be highly skilled in order to accurately carve the tooth anatomy. Another problem with this technique is that special care must be taken to ensure that the patient's mouth is not injured by the exothermal reaction involved in curing the crown material.
As noted, all of the foregoing techniques are undesirably slow. Even the fastest of these techniques generally takes half an hour or more of work for the dentist to fit a temporary or provisional crown to a patient. Additionally, the crowns resulting from the second and third techniques are typically not very durable and are therefore not well-suited for long-term wear. Although the stainless steel shells of the first technique are very durable, it is more difficult to fit stainless steel shells to the patient and to grind the shells' occlusal surfaces to get a comfortable bite.
Accordingly, a need remains in the profession for a way of making temporary and provisional crowns that is quick and accurate, that provides a good fit without substantial trimming, that is durable enough for long-term use, and that is inexpensive.